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copy_to

The copy_to parameter allows you to copy the values of multiple fields into a group field, which can then be queried as a single field.

If you often search multiple fields, you can improve search speeds by using copy_to to search fewer fields. See Search as few fields as possible.

For example, the first_name and last_name fields can be copied to the full_name field as follows:

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "first_name": {
        "type": "text",
        "copy_to": "full_name" 
      },
      "last_name": {
        "type": "text",
        "copy_to": "full_name" 
      },
      "full_name": {
        "type": "text"
      }
    }
  }
}

PUT my-index-000001/_doc/1
{
  "first_name": "John",
  "last_name": "Smith"
}

GET my-index-000001/_search
{
  "query": {
    "match": {
      "full_name": { 
        "query": "John Smith",
        "operator": "and"
      }
    }
  }
}

	The values of the `first_name` and `last_name` fields are copied to the `full_name` field.
	The `first_name` and `last_name` fields can still be queried for the first name and last name respectively, but the `full_name` field can be queried for both first and last names.

Some important points:

It is the field value which is copied, not the terms (which result from the analysis process).
The original _source field will not be modified to show the copied values.
The same value can be copied to multiple fields, with "copy_to": [ "field_1", "field_2" ]
You cannot copy recursively via intermediary fields such as a copy_to on field_1 to field_2 and copy_to on field_2 to field_3 expecting indexing into field_1 will eventuate in field_3, instead use copy_to directly to multiple fields from the originating field.

copy-to is not supported for field types where values take the form of objects, e.g. date_range

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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/copy-to.html